Field of the Technology
The disclosure relates to the field of microgyroscopes and their fabrication.
Description of the Prior Art
Motivated by the proven performance of macro-scale hemispherical resonator gyroscopes (HRG), there has been a growing interest in three dimensional microelectrochemical machined (MEMS) wineglass resonator architectures for use in timing and inertial sensing applications. For example, devices such as rate integrating gyroscopes and mode-matched angular rate gyroscopes rely heavily on the stiffness (Δf) and damping (Δτ) symmetry for high performance operation. Wineglass architectures may enable MEMS-scale integration of these applications due to potential advantages in symmetry, minimization of energy losses and immunity to external vibrations. However, a standalone resonator is insufficient to operate as a gyroscope. Some kind of integrated electrostatic transduction is necessary.
Previously, we demonstrated in-situ electrode structures on MEMS borosilicate glass hemispherical resonators for electrostatic transduction. In these devices, borosilicate glass resonators were fabricated using deep glass etching and XeF2 release of silicon. Characterization using in-situ electrode structures revealed a sub-Hz frequency split on one device and <5 Hz frequency split on multiple devices. Despite the extremely high structural symmetry, the Q-factors were limited to several thousands due to the high impurity content of borosilicate glass and the associated internal dissipation. Because of this reason, research continued on micro-glassblowing of fused silica wineglass resonators.
Fused silica is a desired resonator material due to low amount of impurities within the material and due to its low internal thermo-elastic dissipation, which is required for a high quality factor resonator. A high Q-factor is desired in MEMS vibratory resonators, rate gyroscopes, RF filters, and clocks. Current MEMS fabrication techniques limit the maximum achievable Q-factor by restricting the material choice to few materials and device geometry to two dimensional planar structures. Available materials such as single-crystal silicon have relatively high thermoelastic dissipation and two dimensional planar devices are mostly limited by anchor losses. To take full advantage of the three dimensional wineglass architecture,fabrication techniques with low surface roughness as well as materials with high isotropy and low thermoelastic dissipation (TED) are desired,
We have previously demonstrated stand-alone fused silica glassblown three dimensional resonator structures, however the shells were not released or releasable from the substrate and as a result could not be utilized for resonator or gyroscope operation. It was also missing a transduction mechanism, namely sensing and actuation electrodes to able to operate as a gyro. Integrated sensing and actuation is required for gyro operation.
Wafer-scale fabrication of smooth, symmetric and high aspect ratio three dimensional structures through micro-machining processes remains to be a challenge. This is mainly due to low relative tolerances and low aspect ratios (2.5-D) associated with conventional micro-machining processes Factors such as mold non-uniformity, alignment errors or high surface roughness and granularity of deposited thin films have so far prevented the integration of three dimensional wineglass structures with MEMS techniques. For example hemispherical shells were fabricated by thermally growing oxide in isotropically etched cavities with the lowest as-fabricated frequency split reported at 94 Hz. Diamond hemispherical shells were also fabricated, using micro-crystalline diamond deposition into hemi-spherical molds, a frequency split of ˜770 Hz was reported at ˜35 kHz center frequency. A similar process based on deposition of silicon nitride thin films and isotropic etching of silicon has also been explored and a minimum etch non-uniformity of 1.4% was observed inside the molds due to the crystalline orientation dependent preferential etching in silicon. This effect may be a contributing factor in frequency asymmetry previously observed.
Alternative fabrication techniques include thin film deposition onto high-precision ball bearings blow-molding of bulk metallic glasses, or blow-torch molding of fused silica. Q-factors as high as ˜300,000 were observed on blow-torch molded devices, however relative frequency splits (Δfn=2/fn=2) were on the order of 0.24˜% to 4.49% A ˜2× variation in central frequency was also observed, which was associated with variations in molding duration and the consequent thickness variation.